Device for measuring oxygen content in a gas medium

ABSTRACT

The apparatus includes a sensor capable of delivering a voltage representative of the ratio between a reference oxygen pressure and the oxygen pressure in a volume of the sensor and provided with electrodes ( 12, 14 ) via which it is possible to pass a pumping current that controls said oxygen pressure in the volume, and monitoring and control means including a digital controller ( 26 ) receiving said representative voltage on an input and suitable for delivering the pumping current. The monitoring and control means deliver the pumping current in the form of a current that varies continuously and progressively, without interruptions, governed by the digital controller in such a manner as to servo-control the input voltage to a determined value.

BACKGROUND OF THE INVENTION

The invention relates to apparatuses for measuring the oxygen content ofa gaseous medium, the apparatuses being of the type comprising:

a sensor capable of delivering a voltage representative of the ratiobetween a reference oxygen pressure and the oxygen pressure in a volumeof the sensor which communicates with the gaseous medium via a porouswall; and

monitoring and control means enabling a pumping current to be deliveredto cause oxygen to migrate away from or into said volume.

Numerous apparatuses of that type are already known, such as thosedescribed in EP-A-0 507 149 and U.S. Pat. No. 4,932,238. The sensor hasat least one sensitive element constituted by a solid electrolyte plateof a type that allows oxygen ions to migrate, the plate being placedbetween two porous electrodes.

Such a sensor can be the subject of numerous embodiments. FIG. 1 is adiagram of a sensor that can be considered as having two cells. A firstcell referred to as a “pumping” cell 10 _(p) is sandwiched between twoelectrodes 12 and 14. The pumping cell 10 _(p) is fixed to a second cell10 _(s), referred to as the “sensitive” cell, via a porous intermediatesheet so as to define a volume 18. Oxygen in the gaseous medium tends topenetrate into the volume 18 so as to bring the oxygen partial pressuresinto equilibrium. The passage of a current I_(p) through the pumpingcell tends to cause the oxygen contained in the volume to migrate, andthus to maintain the partial pressure therein at a determined value. Afinal plate 20 can be placed in contact with the sensitive element 10_(s) and can be made of the same material so as to deliver a constantreference pressure; its usefulness appears below.

Between the electrodes on either side of the sensitive element 10 _(s)there thus appears a measurement voltage V_(s) representative of theratio between the partial pressures of oxygen in the volume 18 and inthe plate 20 in contact with the cell 10 _(s). Appropriate solidelectrodes, and in particular of doped zirconium oxide or zirconia, havecharacteristics such that the voltage V_(s) varies in substantiallylogarithmic manner with oxygen partial pressure in the volume 18.Conventionally, the current I_(p) is controlled so as to maintain V_(s)at a constant value, in which case I_(p) is representative of the oxygenpartial pressure in the gaseous medium. A heating resistor 21 serves toraise the cells to a suitable temperature.

In another embodiment, that can be described as a single cell embodimentand as shown in FIG. 2, the volume 18 is defined solely by the porousintermediate sheet 16 and by the cell 10 _(p). The reference oxygenpartial pressure is then that of atmospheric air, which is in contactwith the cell 10 _(p). Under such circumstances, variation of I_(p) as afunction of V_(p)for different oxygen partial pressures has the generalappearance shown in FIG. 3. Insofar as it is desired to remain at alltimes within a rectilinear portion of the characteristic, the value towhich V_(p)is servo-controlled must depend to some extent on the partialpressure of oxygen in the gas and on the impedance of the cell.

A major application of the invention lies in determining the air/fuelratio admitted into an internal combustion engine on the basis of thecomposition of the exhaust gas, and more particularly on the basis ofthe partial pressure of the residual oxygen in the exhaust gas.

Until now, use has been made above all of sensors of the kind shown inFIG. 1. Often the monitoring and control means are constituted by ananalog loop for servo-controlling V_(s) to a constant value, associatedwith a microcontroller which deduces the oxygen partial pressure and theinstantaneous richness of the mixture from the value of the currentI_(p). That solution suffers from drawbacks. The accuracy with whichrichness is measured is limited by the accuracy with which I_(p) ismeasured. Embodiment in hard-wired form reduces possibilities ofadjustment and matching.

SUMMARY OF THE INVENTION

It is an object of the invention to provide improved apparatus anotherobject is to satisfy practical requirements better than previously knownapparatuses, at low cost, and enabling richness to be measuredaccurately.

To this end, the invention provides in particular apparatus of theabove-defined type in which the monitoring and control means have adigital controller receiving said voltage on its input and deliveringthe pumping current to the sensor in the form of a current that variescontinuously and progressively without interruptions, and governed bythe digital controller in such a manner as to servo-control said voltageon a determined value. The architecture can be such that themicrocontroller has means for acting on the current I_(p), therebyproviding greater flexibility in adjusting the servo-control, goodprotection for the sensor, and finer management of putting into actionand of degraded mode.

An advantageous solution consists in providing a digital controller thatdelivers a control voltage which is time modulated (e.g. by pulse widthmodulation) and in providing a lowpass filter and a current generator atthe input of the sensor. Such modulation is easily performed by means ofa digital controller. The presence of the filter avoids the need toapply interrupted pulses to the sensor which would reduce its lifetime.

The sensors have integrating behavior. It is advantageous to compensateit by subjecting said voltage representative of the ratio to processingthat introduces a derivative component, prior to application to thedigital controller. This increases the signal-to-noise ratio and enablesthe controller to reduce response time. In general, an amplifier circuitis provided to amplify the proportional component, and also thederivative component if present, prior to application to the digitalcontroller.

The invention is applicable to apparatuses in which the sensor has asingle cell separating the volume which communicates with the gaseousmedium from a zone in which there exists a reference pressure; therepresentative voltage is then taken from the two electrodes on eitherside of the cell, and the pumping current is applied to the same cell.Nevertheless, the invention is more usually applied to apparatuses inwhich the sensor has two cells. Such a sensor has a volume defined by aporous intermediate sheet, by a first cell or “pumping” cell separatingthe volume from a zone occupied by the gas whose oxygen partial pressureis to be measured, and by a second cell or “sensitive” cell in contactwith the reference pressure, with the representative voltage being takenfrom the electrodes on either side of the second cell. The pumpingcurrent then passes through the two electrodes on either side of thepumping cell.

In general, it can be considered that the main advantages of theapparatus stem from the fact that the digital controller has completecontrol over the pumping current I_(p), both in normal servo-controlmode and during transient stages (starting up, protecting the probe,diagnostics, . . . ), and that it serves to produce the current ratherthan to read back a current that is produced by external means as inpresent architectures.

Also, the apparatus:

firstly takes advantage of an analog portion that delivers an amplifiedsignal on the basis of the sensor voltage V_(s), which signalfortunately has a derivative component of accuracy that is not degradedby analog-to-digital conversion situated downstream from the preparationof said derivative component; and

also makes it possible to produce the current I_(p) by time modulationin the digital controller, which modulation, in association with alowpass filter and a current controller governed by the filtered commanddelivers a pumping current I_(p) that is variable continuously,progressively, and without interruptions, under the control of thedigital controller.

Said current I_(p) is then produced in a form that does not require anintegrated digital-to-analog converter, even though such a converter canconstitute a solution when the digital controller has one.

There exist stages in the operation of the apparatus during which thesensor would run the risk of being damaged if normal operating mode wereto be maintained. For example, the normal operating conditions forapparatus having two cells consist in maintaining the voltage V_(s) asmeasured on the second cell at a reference value and in deducing theoxygen content from the value of the pumping current I_(p). That mode ofoperation requires the probe to be at a sufficiently high temperature,generally in the range 650° C. to 90° C. The sensor is raised to thistemperature by a heating resistor. During an interval of time startingfrom the beginning of heating, the apparatus is inoperative because itstemperature is too low. Nevertheless, it is desirable to obtainmeaningful measurements as soon as possible, even if they are lessaccurate, together with an indication of the instant from which themeasurements can be used.

In addition, various operating conditions make the normal mode ofoperation liable to degrade the sensor because they lead to too high avoltage V_(p) across the pumping cell.

In an advantageous embodiment of the invention, and under conditionsthat make the normal mode inappropriate, the apparatus can obtainmeasurements that are degraded, but nevertheless of use.

For this purpose, the monitoring and control means can include means forswitching between servo-controlling the voltage of the sensitive cell ona constant value and limiting the voltage of the pumping cell to aceiling value, suitable for protecting the sensor.

In particular, the controller can be designed to initialize operation bycontrolling the pumping current I_(p) so as to limit the voltageV_(p)across the pumping cell to a maximum value that is compatible withprotecting the cell, and then in repeating the following sequence:

the pumping current is servo-controlled to the voltage applied to thesensitive cell, while verifying that the voltage across the pumping cellremains below the ceiling; and

the pumping current is again governed so that it is compatible with theceiling insofar as the ceiling is exceeded during the servo-control ofthe voltage V_(s) applied to the sensitive cell.

More generally, the digital controller can be designed to use thevoltages measured across the terminals of both cells and to take thepumping current I_(p) into account in order to perform additionalfunctions, such as:

estimating the temperature of the probe, in order to determine theinstant at which it becomes usable;

diagnosing aging; and

operating in degraded mode, with servo-control being applied to thevoltage V_(p) of the pumping cell instead of to the voltage V_(s).

When operating in degraded mode, V_(p) need not be servo-controlled to afixed value, but to a value that is selected so as to take account ofother parameters or of the function to be performed, such ascalibration, diagnosis, or operating in degraded mode due to abnormalconditions.

The above characteristics and others will appear better on reading thefollowing description of particular embodiments given as non-limitingexamples. The description refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2, already mentioned above, show respectively a two-cellsensor and a one-cell sensor;

FIG. 3 is a graph in which the curves show pumping current variation asa function of voltage across the pumping cell for various partialpressures of oxygen in the surrounding atmosphere;

FIG. 4 is a block diagram of apparatus of the invention;

FIG. 5 is a diagram for showing the way functions are shared betweenhardware and software in a particular embodiment; and

FIG. 6 shows how the resistance of the pumping cell varies as a functionof temperature for a rich mixture and for a mixture that is relativelylean.

DETAILED DESCRITION OF PREFERRED EMBODIMENTS

The apparatus whose general structure is shown in FIG. 4 comprises asensor 22 whose structure is as shown in FIG. 1. That is why elementscorresponding to elements in FIG. 1 are given the same referencenumerals.

The monitoring and control means associated with the sensor 22 can bethought of as having a “hardware” or hard-wired portion 24 and a digitalcontroller 26 constituting a “software” portion.

In normal and stabilized operation of the apparatus, the monitoring andcontrol means receive the voltage across the terminals of the sensitivecells 10 _(s) which appears between terminals V_(s)− and V_(s)+, andthey deliver a pumping current to the sensor, which current flowsbetween the terminals I_(p)+ and I_(p)−. Ground connections and powersupplies are not shown in FIG. 4 in order to simplify the figure.

The voltage V_(s) is processed in the hardware portion 24 by an analogcircuit 28 for introducing a derivative component and for amplificationwith gain G. The signal as processed in this way is applied to thedigital controller 26. It is digitized therein by a digital-to-analogconverter (DAC) which receives the signal on an input 30. In anapplication of the apparatus to controlling an internal combustionengine, a converter operating on eight to ten bits is generallysufficient. The controller includes software, represented at 32, forgoverning the servo-control V_(s) on a constant value, which value canbe fixed by a generator 34 that generates a reference voltage V_(r). Theservo-control is advantageously selected so that the set point value ofV_(s) is substantially in the middle of a linear portion of theI_(p)(V_(s)) characteristic.

The software 32 is designed to deliver a signal that is fed to thecircuit 36 that delivers a time-modulated voltage signal, generallyusing pulse width modulation (PWM). Such digital components and programsserving to transform a digital input signal into a signal modulated inthis way are commonly available.

The pulse width modulated signal, of variable duty ratio, is applied toa lowpass filter 38 so as to transform it into a signal that is notinterrupted, which signal is applied to a direct current generator 40that delivers the current I_(p). Given that the sensors generallypresent dispersion in the sensitivity of the characteristic of pumpingcurrent I_(p) as a function of richness, the current generator 40 can bedesigned to take account of a compensation resistor 62 integrated in thesensor to normalize the characteristic of I_(p) as a function ofrichness. Still in the special case of apparatus designed to beincorporated in an engine control system, pulse width modulation encodedon twelve bits provides sufficient resolution. The least significant bitcan correspond, for example, to a change in the current I_(p) of theorder of 3 μA.

As mentioned above, certain stages of operation make it desirable toservo-control the voltage V_(p) rather than the voltage V_(s). To makethis mode of operation possible, the digital controller 26 may includean analog-to-digital converter (ADC) whose analog input 42 receives thevoltage VP taken from the terminal I_(p)+.

Satisfactory operation of the sensor requires the cells to be at anappropriate temperature, generally lying in the range 650° C. to 900° C.This temperature can be maintained by controlling the heat dissipated ina heater resistor 21. This control can include servo-control making useof the digital controller 26. In the case shown diagrammatically in FIG.4, the resistor 21 is powered by an analog power control and currentmeasuring circuit 44 connected to an analog input of ananalog-to-digital converter 46 of the controller which may optionally bethe same ADC as that provided with the inputs 30 and 42. The software 48of the controller performs the servo-control function and the functionof controlling an output modulator 50 which delivers a pulse widthmodulated signal controlling the circuit 44.

The way in which temperature servo-control operates for the purpose ofmaintaining the cells at a determined temperature can be as follows.

Initially, laws concerning variation in the resistance of the pumpingcell as a function of temperature, for various oxygen concentrations(corresponding to various different degrees of mixture richness whenfeeding an engine) are loaded into the software portion 32 of thedigital controller 26. In normal operation, it is known that a leanmixture is desirable, which leads to the presence of residual oxygen inthe exhaust gas.

The software 32 then includes a program that periodically causes acurrent increment ΔI_(p)to be applied to the current I_(p). Thisincrement gives rise to a variation ΔV_(p) in the voltage V_(p) asmeasured on the output I_(p)+. The controller 26 can determine thisvariation ΔV_(p) by taking the difference between two successive digitalvalues that appear on the output of the input ADC 42. The resistance Rof the pumping cell is given by the ratio ΔV_(p)/ΔI_(p). Temperature isdeduced therefrom by referring to a lookup table, stored in digital formin the software 32. The corresponding information can be transmitted tothe governing software 48 for modifying the current flowing through theheater resistor 21 accordingly.

For a sensor that has only one cell, a voltage E_(p) to be regulated ona value of about 450 mV, for example, on the basis of known values forV_(p) and I_(p). Use is then made of an estimate of the resistance R ofthe single cell as given by:

R=ΔV _(p) /ΔI _(p)

The apparatus of FIG. 4 also makes it possible to protect the sensorwhile continuing with degraded operation, by limiting the voltage V_(p).For this purpose, the digital controller compares V_(p) with a storedmaximum value, which value can be fixed or which can vary as a functionof external parameters. If V_(p) tends to exceed the threshold value,the servo-control mode is modified. The current I_(p) is controlled tomaintain the voltage V_(p) at the maximum authorized value. Thisgenerally implies returning at regular intervals to an attempt atservo-controlling the pumping current to the voltage V_(s), and thenimmediately returning to servo-controlling V_(p) in the event of V_(p)exceeding the acceptable value.

Operation of the apparatus can be initialized (sensor cold) as follows,at least when the apparatus is integrated in an engine control system.Under such circumstances, starting is performed using a mixture that isrich, i.e. with a law for variation of impedance as a function oftemperature as given by dashed lines in FIG. 6. The digital controller26 is designed to attempt initially to operate the probe withservo-control of the voltage V_(p) and with evaluation of the resistanceR at a high rate. Under such circumstances, meaningful measurements,although less accurate than those taken in normal operation, can beobtained and validated as soon as the measured resistance R shows thatthe temperature has reached a first value, e.g. in the range 550° C. to650° C. A changeover to attempting to servo-control V_(s) can beprogrammed on reaching a determined value for the resistance R.

The way in which functions are shared between the hardware portion andthe software portion can be as shown in FIG. 5, where functions aredesignated by the same reference numerals as the correspondingcomponents in FIG. 4. The interface between the software portion and thehardware portion comprises the inputs 30 and 42 and the input to theanalog-to-digital converter 46; the converters can have 10-bit outputs.The interface also comprises the pulse width modulator circuits 36 and50. The modulator circuit 36 can operate at a frequency of 5 kHz and canhave a 12-bit output. The modulator circuit 50 will generally operate ata much lower frequency, e.g. about 30 Hz.

The heating resistor can be regulated in conventional manner so as tomaintain its resistance at a determined value fixed by a set point 60. Aproportional-integral-derivative type stabilization circuit 52 can beprovided.

As mentioned above, the controller includes a program for testing thetemperature of the sensor and for testing the value of V_(p), acting at56 to direct regulation either to maintaining V_(s) at a set point valuefor V_(s) (input 2), or maintaining V_(p) at a set point value (input1).

What is claimed is:
 1. Apparatus for measuring the oxygen content of amedium, the apparatus comprising: a sensor having a porous wall forcommunicating a volume thereof with said medium arranged for deliveringa voltage representative of the ratio between a reference oxygenpressure and oxygen pressure in a fraction of said medium present in avolume of the sensor, and provided with electrodes enabling a pumpingcurrent to be passed for controlling said oxygen pressure in the volume;and monitoring and control means including a digital controllerconnected to receive said representative voltage on an input thereof andarranged for delivering the pumping current; the apparatus beingcharacterized in that the monitoring and control means deliver pumpingcurrent in the form of a current that varies continuously andprogressively, without interruptions, governed by the digital controllerto servo-control said representative voltage and to maintain saidrepresentative voltage at a determined value.
 2. Apparatus according toclaim 1, characterized in that the monitoring and control means includea circuit that introduces a derivative component prior to saidrepresentative voltage being applied to the digital controller. 3.Apparatus according to claim 1, characterized in that the monitoring andcontrol means include a circuit introducing gain to a proportionalcomponent of said representative voltage, and gain to a derivativecomponent if present, prior to application to the digital controller. 4.Apparatus according to claim 1, characterized in that the sensorcomprises a single cell separating the volume of a zone in which thereference pressure exists, in that the representative voltage is takenfrom two electrodes on either side of the cell, and in that the pumpingcurrent is applied to the same cell.
 5. Apparatus according to claim 1,characterized in that the sensor has a volume defined by a porousintermediate sheet and by a first or “pumping” cell separating thevolume from a zone occupied by the gas where the oxygen partial pressureis to be measured, and a second or “sensitive” cell in contact with thereference pressure, in which the representative voltage is taken fromelectrodes on either side of the second cell, and in that the pumpingcurrent passes through two electrodes on either side of the first cell.6. Apparatus according to claim 5, characterized in that the monitoringand control means comprise means for changing-over fromservo-controlling the voltage of the sensitive second cell to a constantvalue to limiting the voltage of the pumping cell to a predeterminedceiling value for protecting the sensor.
 7. Apparatus according to claim5, characterized in that the controller is designed to initializeoperation by controlling the pumping current to limit the voltage(V_(p)) across the pumping cell to a maximum value compatible withprotecting the sensor, and in repeating a pumping current servo-controlsequence on the voltage (V_(s)) across the second cell while verifyingthat the voltage (V_(p)) remains below a ceiling, and returning togoverning the pumping current so that it is compatible with the ceilingof V_(p) insofar as the ceiling is exceeded by servo-control governingof V_(s).
 8. Apparatus according to claim 5, characterized in that saidmonitoring and control means include means for evaluating the resistanceof the pumping cell, having means for periodically applying a temporaryincrement to the pumping current and for measuring the correspondingchange in the voltage of said cell.
 9. Apparatus according to anypreceding claim, characterized in that the controller also performsengine control functions.
 10. Apparatus for measuring an oxygen contentin exhaust gas of an internal combustion engine, comprising: a sensorfor delivering a voltage representative of a ratio between a referenceoxygen pressure and oxygen pressure in a volume of the sensor,communicating with said exhaust gas by a porous wall provided withelectrodes enabling a pumping current to be passed for pumping oxygenout of the volume and controlling said oxygen pressure in the volume;and monitoring and control means including: digital controller meansconnected to receive said representative voltage on an input thereof andarranged to deliver a time-modulated control voltage, and a seriesarrangement of a lowpass filter and a current generator connected toreceive said time modulated control voltage and arranged for deliveringsaid pumping current to said electrodes of the sensor in the form of acurrent having steady variations without time interruptions, saiddigital controller means being programmed to continuously adjust saidtime modulated control voltage for maintaining the representativevoltage at a predetermined value, whereby said pumping current is arepresentation of said oxygen content.
 11. Apparatus according to claim10, characterized in that the digital controller delivers at its outputa signal for governing the monitoring and control means, which signal isconstituted by a pulse width modulated voltage.